I like to write letters on vellum, so I have plenty of experience working with it. I decided to write a few tips for those who are working with it for the first time, I hope they will come in handy.
Firstly, let's choose a vellum type (see the photos below for reference).
Number one is vellum for ink. It's glossy and it's hard to write on, so itisn't a good fit. Under number two is a vellum for pencil. It's matte and very transparent - the best choice on my opinion. The other two vellum types are for baking; they can have glossy and matte sides, so if you are going to use them for writing or making envelopes, use matte side.
The first two types can be purchased at any stationery (craft) store, and the third and fourth in the any supermarket.
Let's take vellum for pencil. It is easy to write on and it is fairly transparent, so we will use it. No need to draw lines - just apply a sheet of vellum paper to any lined paper and affix it with any way you like (using paper tape or paper clips).
I advise you to check how your pen will write on a vellum paper, before writing a letter. Try to write a test phrase on a sheet of vellum. You may notice that ballpoint pens rarely work well on vellum, and gel pen ink can be easily smudged. I like to write on vellum with liners, they are thin, work good and do not smudge.
If you still have questions, contact me!